Is Hammie a Syrian hamster? The ideal time for a hamster to be re-homed is about a month old, so Im assuming that pet stores get their young hams at about this age. They still would look fairly small at this stage, their eyes often appear a bit smaller but bright and as they age they start to fatten up and fill out, so I would just assume your hamster was about a month old and just try and figure out from what day you got it to what date it was a month from then to determine a age.
So maybe around January 4th to the 9th your hamster may have been born?I really such with math so someone correct me if im wrong :P